Polyoxyalkylenes, also known as acetal or polyacetal, demonstrate excellent mechanical properties, chemical resistance and relatively good friction properties. As such, polyoxyalkylenes have been used in many diverse applications from plumbing parts to electrical switches. A primary use of polyoxyalkylenes is in the formation of moving parts in tribological systems in which one surface bears against another surface causing friction and wear to each surface. For example, polyoxyalkylenes are used as appliance parts such as gears, cams and rollers and are used in moving vehicles as gears, automobile window lift mechanisms and crank parts.
It is known in the art to provide good wear, friction and melt flow properties to polyoxyalkylene compositions by using fluorinated hydrocarbons, such as polytetrafluoroethylene (PTFE), as lubricant additives. However, in many areas of the world, particularly in Europe, fluorinated materials are creating increasing concern due to the potential of these substances to act as environmental hazards. As a result, it is becoming increasingly important to develop alternative, non-fluorinated lubricants for polyoxyalkylene compositions which are capable of delivering equivalent or improved wear, friction and melt flow properties.
U.S. Pat. Nos. 4,737,539 and 5,216,079 teach that polyolefins of a molecular weight less than 500,000, alone or in a blend with PTFE, can act as internal lubricants for various polymers, including polyoxymethylene. In addition, U.S. Pat. Nos. 4,737,539 and 4,877,813 teach use of PTFE and polyamide fibers to stabilize the coefficient of friction in certain resin compositions including polyoxymethylene.
While these compositions, and other prior art PTFE/polyoxyalkylene compositions, have achieved reasonably good wear properties, particularly when PTFE is added, a fluorocarbon-free composition has yet to be developed that safely and economically provides friction, wear and melt flow properties to polyoxyalkylene compositions which meet or exceed the values of those properties in a polyoxyalkylene composition containing PTFE. In addition, while prior art compositions including PTFE show good properties in tribological systems in which a surface formed of a polymeric composition containing a polyoxyalkylene bears against a metal surface or a different polymeric surface, those same properties have not been sufficiently demonstrated in tribological systems in which a surface formed of a polyoxyalkylene composition bears against another surface formed of the same material.
As a result, a need in the art exists for a self-lubricating, fluorocarbon-free polyoxyalkylene composition which provides equivalent or improved wear, friction and melt flow properties to those exhibited by fluorocarbon-containing polyoxyalkylene compositions. In addition, it is desirable to develop such a composition which not only exhibits such properties in plastic against metal tribological wear systems, but also in plastic against plastic tribological wear systems in which the two plastic surfaces are the same.